1. Field of the Invention
The present invention relates to an illumination optical system and an image projection device, and more specifically to an image projection device having, for example, a digital micromirror device or a liquid crystal display (LCD) as a display element and an illumination optical system for illuminating an image display surface of the display element in the image projection device.
2. Description of Related Art
A surface emitting semiconductor laser array has received attention as a light source for a projector. This semiconductor laser array often has very flat light source arrangement, for example, 1 or 2 rows by 10 numeric sequences. Thus, when illumination is achieved with such a laser array light source, NA (numerical aperture) of illumination light greatly varies depending on directions. On the other hand, an optical system of, for example, a projection lens used in a projector has isotropic NA.
With illumination light with flat NA as described above, adjusting the optical system to smaller NA results in failure to transmit a larger NA component of the illumination light. On the other hand, adjusting the optical system to larger NA leads to upsizing of the optical system, which results in poor total efficiency. Moreover, adjusting the optical system to the larger NA of the illumination light with appropriate NA (for example, NA=0.2) greatly reduces the smaller NA (for example, NA=0.025) of the illumination light, resulting in failure to provide resolution for the smaller NA of a projected image. As described above, the use of a laser array light source presents a problem of flat NA distribution.
Patent Document 1 listed below provides an example of an illumination optical system using a laser array light source.
Patent Document 1: U.S. Pat. No. 5,704,700
In the illumination optical system described in Patent Document 1, a pair of lens arrays are used which permit homogeneous illumination having rectangular distribution with a beam, from the laser array light source, having Gaussian distribution. When the laser array light source radiates a flat beam, with a method of illumination by use of the pair of lens arrays, the lens arrays are arranged in a flat region in accordance with flat beam distribution. The flat region occupied by the lens arrays at this point has NA distribution of this illumination optical system. Therefore, the NA distribution does not become isotropic, although this is not at all described. Moreover, with the lens array method, superposition of rectangular illumination light and edge focusing performance in an illuminated region are insufficient, which requires a sufficient illumination margin region to be ensured. As a result, loss of an amount of light occur. Moreover, a too broad illumination margin region causes ghost occurring outside the screen, which is not preferable in a projector for a home theater in particular.